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Supermassive black hole found guzzling cold clumpy gas

An artist's impression of a small cluster of giant gas clouds raining on the black hole in the heart of the Abell 2597 Brightest Cluster Galaxy.

NRAO / AUI / NSF / Dana Berry / SkyWorks / ALMA / ESO / NAOJ / NRAO

For the first time, a supermassive black hole has been spotted about to binge on a massive meal of cold, "chaotic" gas clouds – not a smooth, steady flow of hot gas as previous models predicted.

An international team of astronomers, led by Grant Tremblay from Yale University in the US, watched as cool, clumpy clouds of gas clouds rained down on a supermassive black hole, 300 million times the mass of the Sun, in the centre of the Abell 2597 Brightest Cluster Galaxy a billion light-years away.

"Although it has been a major theoretical prediction in recent years, this is one of the first unambiguous pieces of observational evidence for a chaotic, cold rain feeding a supermassive black hole," Tremblay says.

The work, published in Nature, offers insights in how supermassive black holes, found in the centre of large galaxies, devour fuel and grow.

The process, called "hot mode" accretion, occurs when hot gas spirals in slowly from the surrounding galaxy. But do black holes really only eat hot meals?

Tremblay and his colleagues used the Atacama Large Millimeter/submillimeter Array (ALMA) telescope in Chile to peer at movements of carbon monoxide gas around the biggest, brightest galaxy in a cluster known as Abell 2597.

Between the galaxies is an atmosphere of hot, ionised gas. But the researchers noticed something that bucked traditional theories about supermassive black hole growth.

The background image (blue) is from the Hubble Space Telescope. The foreground (red) is ALMA data showing carbon monoxide gas in and around the galaxy. The pull-out box shows "shadow" of a cold cloud of molecular gas raining on the black hole.

They saw three massive clouds of cold gas, each a million suns in mass and tens of light-years across, zooming towards the Abell 2597 Brightest Cluster Galaxy as fast as a million kilometres per hour.

"This very, very hot gas can quickly cool, condense, and precipitate in much the same way that warm, humid air in Earth's atmosphere can spawn rain clouds and precipitation," Tremblay says. It's these newly formed clouds that "rain down" on galaxies, fuel star formation – and feed the supermassive black hole.

The black won't just feed on cold gas – the team saw hot ionised gas in the innermost regions of the galaxy too.

It's like instead of slowing sipping on spoonfuls of hot porridge, the black hole will also gulp down bowls of cold, coagulated stuff.

And even though the cold clouds were tens of light-years across, they were still too small to directly measure with the telescope. Luckily, they cast billion-light-year-long "shadows" towards Earth, and it was these ALMA used to track their movements.

There are likely more gassy clumps near the black hole, the researchers write, and will use ALMA to search for more "rainstorms" to see if it's a common occurrence.

Massachusetts Insitute of Technology astronomer and co-author of the paper Michael McDonald admits the team "got very lucky" when they spotted three clouds: "We could probably look at 100 galaxies like this and not see what we saw just by chance.